Hydromechanical transmission for agricultural tractors

ABSTRACT

The transmission comprises: an input shaft t (IS) which can be coupled to a prime mover of the tractor and extends along a first direction (x 1 ) aligned with the shaft of the prime mover and with a shaft (S 1 ) of the power take-off of the tractor; an output shaft (OS); a hydrostatic unit (HU) including a pump (P) driven by the prime mover shaft and a motor (M) driven by the pump (P), wherein the pump and motor are positioned in line along a second direction (x 2 ) spaced transversely downwards from the first direction (x 1 ); an epicyclic torque splitter unit (TSU), positioned in line with the hydrostatic unit (HU) and including a first and a second input shaft (S 3 , S 4 ), coupled to the motor (M) of the hydrostatic unit (HU) and to the prime mover, respectively, and at least a first and a second output shaft (S 5 , S 6 ), the rotational speeds of the first and second output shafts (S 5 , S 6 ) varying, respectively, in a first and a second range (VDI-VD 2 , VC 1 -VC 2 ; VD 1 -VD 2 , VB 1 -VB 2 ) adjacent to each other, at high speed and low speed respectively, as the rotational speed of the first input shaft (S 3 ) varies between a maximum value (rpmA; rpmC) and a minimum value (−rpmA; −rpmC); a clutch unit (CU), positioned in line with the hydrostatic unit (HU) and with the torque splitter unit (TSU) and arranged to couple the output shaft (OS) of the transmission selectively to the first or second output shaft (S 5 , S 6 ) of the torque splitter unit (TSU), in such a way as to provide a pair of forward operating ranges (“transport” and “work”), at high and low speed respectively; and a reversing unit (RU) positioned in line with the hydrostatic unit (HU) the torque splitter unit (TSU) and the clutch unit (CU), and arranged to provide a reverse operating range (“reverse”).

The present invention relates to a continuously variable hydromechanicaltransmission for agricultural tractors, particularly for specialtractors to be used in orchards or the like.

Tractors of this type require transmissions which are extremely compact,both in terms of height and in the direction of the width of thevehicle, since the transmission is housed under the driving area of thevehicle, between the driver's footrests. Hydromechanical transmissionsof the known type are hardly suitable for fitting on these agriculturaltractors, since they have large overall transverse dimensions.

The object of the present invention is therefore to provide ahydromechanical transmission for agricultural tractors, particularly forspecial tractors to be used in orchards or the like, which has astructure which is as compact as possible.

This and further objects are fully achieved according to the presentinvention by means of a continuously variable hydromechanicaltransmission having the characteristics set forth in claim 1. Furtheradvantageous characteristics of the invention are specified in thedependent claims.

Briefly, the invention is based on the idea of providing a continuouslyvariable hydromechanical transmission comprising:

-   -   a hydrostatic unit including a variable displacement pump which        can be driven by the prime mover shaft of the tractor and a        fixed displacement motor driven by the pump, wherein the pump        and motor are positioned in line along a direction spaced        transversely downwards from the axis of the prime mover shaft        and from the axis of the shaft of the power take-off of the        tractor;    -   an epicyclic torque splitter unit, positioned in line with the        hydrostatic unit and including a first and a second input shaft,        coupled to the motor of the hydrostatic unit and to the        tractor's prime mover respectively, and at least a first and a        second output shaft;    -   a clutch unit, positioned in line with the hydrostatic unit and        with the torque splitter unit and arranged to couple the output        shaft of the transmission selectively to the first or second        output shaft of the torque splitter unit, in such a way as to        provide a pair of forward operating ranges;    -   a reversing unit, positioned in line with the hydrostatic unit,        the torque splitter unit and the clutch unit, and arranged to        provide a reverse operating range.

The characteristics and advantages of the present invention will appearfrom the following detailed description, provided purely by way ofnon-limitative example, with reference to the attached drawings, inwhich:

FIG. 1 is a schematic illustration of a continuously variablehydromechanical transmission according to a first embodiment of thepresent invention;

FIG. 2 is the Ravigneaux speed graph relating to the torque splitterunit of the transmission of FIG. 1;

FIG. 3 is a schematic illustration of a second embodiment of ahydromechanical transmission according to the invention;

FIG. 4 is the Ravigneaux speed graph relating to the torque splitterunit of the transmission of FIG. 3;

FIG. 5 is the Ravigneaux speed graph relating to the reversing unit ofthe transmissions of FIGS. 1 and 3; and

FIG. 6 is a graph showing an operating characteristic, in the diagram ofspeed of the tractor/number of revolutions of the output shaft of thehydrostatic unit, which can be obtained with a transmission according tothe invention.

With reference first to FIG. 1, a continuously variable hydromechanicaltransmission according to a first embodiment of the present invention isgenerally indicated T.

The transmission T has an input shaft IS which can be coupled by meansof a clutch Ci to an internal combustion prime mover (not shown) of anagricultural tractor. The input shaft IS is coaxial with a shaft S1 ofthe power take-off of the tractor, which extends in the longitudinaldirection of the tractor along an axis x1.

The transmission T comprises a hydrostatic unit HU consisting of avariable displacement pump P and a fixed displacement motor M,positioned in line along an axis x2 parallel to, and offset downwardsfrom, the axis x1. Clearly, the motor M could also be of the variabledisplacement type.

The transmission T also comprises a mechanical torque splitter unit TSUof the epicyclic type, positioned in line with the hydrostatic unit HU.

The input shaft IS carries a first gear wheel G1 which meshes with agear wheel G2 carried by an input shaft S2 of the hydrostatic unit HU.The pump P of the hydrostatic unit can therefore be driven by thetractor's prime mover by means of this first gearing G1-G2.

The torque splitter unit TSU includes:

-   -   a first input sun gear A drivingly connected to an output shaft        S3 of the hydrostatic unit HU, in other words to a first input        shaft of the unit TSU;    -   a second input sun gear B drivingly connected to a second input        shaft S4 of the unit TSU, made in the form of a hollow shaft in        which the shaft S3 is inserted;    -   an output sun gear D drivingly connected to a first output shaft        S5 of the unit TSU; and    -   a double planet carrier C which carries, on an inner        circumference, three sets, spaced apart angularly at 120° (only        one of which is shown in FIG. 1), of three planet gears sb, sa,        sd drivingly connected for rotation to each other, and, on an        outer circumference, three planet gears se, spaced apart        angularly at 120° (only one of which is shown in FIG. 1).

Each of the planet gears sb meshes with the sun gear B, each of theplanet gears sa meshes indirectly with the sun gear A via the planetgears se, and each of the planet gears sd meshes with the sun gear D.The planet carrier C is drivingly connected to a second output shaft S6of the unit TSU, made in the form of a hollow shaft in which the firstoutput shaft S5 is inserted.

The input shaft IS also carries a second gear wheel G3 which meshes witha gear wheel G4 carried by the input shaft S4 of the torque splitterunit TSU. The unit TSU therefore receives torque from the tractor'sprime mover through this second gearing G3-G4, as well as from thehydrostatic unit HU (through the shaft S3 and the sun gear A).

The transmission T also comprises a reversing unit RU, of the epicyclictype, positioned in line with the hydrostatic unit HU and with thetorque splitter unit TSU along the longitudinal axis x2.

The reversing unit RU includes:

-   -   a first sun gear Ar drivingly connected to a first input shaft        S7 of the unit RU, made in the form of a hollow shaft drivingly        connected for rotation to the second output shaft S6 of the        torque splitter unit TSU;    -   a second sun gear Br drivingly connected both to a second input        shaft S8 and to an output shaft OS of the unit RU, wherein the        second input shaft S8 is inserted in the first hollow input        shaft S7 and the output shaft OS also forms the output shaft of        the transmission T, which imparts the motion to the rear wheels        of the tractor through a bevel gear G5 and to the front wheels        through a gearing G6-G7; and    -   a double planet carrier Cr, which carries on an inner        circumference three sets, spaced apart angularly at 120° (only        one of which is shown in FIG. 1), of two planet gears sar and        sbr, drivingly connected for rotation to each other, and, on an        outer circumference, three planet gears sdr, spaced apart        angularly at 120° (only one of which is shown in FIG. 1).

Each of the planet gears sar meshes with the sun gear Ar, and each ofthe planet gears sbr meshes indirectly with the sun gear Br via acorresponding planet gear sdr. The planet carrier Cr is mountedrotatably with respect to the input and output shafts of the unit RU andcan be locked by means of a braking device BD of a per-se-known type.

A clutch unit CU is interposed between the torque splitter unit TSU andthe reversing unit RU, and includes:

-   -   a first clutch Cw for coupling the first and second input shaft        S7, S8 of the reversing unit RU to provide a first low-speed        “work” operating range of the transmission, as will be explained        in detail below; and    -   a second clutch Ct for coupling the first output shaft S5 of the        torque splitter unit TSU to the second input shaft S8 of the        reversing unit RU to provide a second higher-speed “transport”        operating range of the transmission, as will be illustrated in        the rest of the description.

A set of speed sensors is also provided, comprising, in the illustratedembodiment, a first sensor ss1 associated with the output shaft S3 ofthe hydrostatic unit HU, a second sensor ss2 associated with the gearwheel G3 carried by the input shaft IS of the transmission T and a thirdsensor ss3 associated with the gear wheel G6 carried by the output shaftOS of the transmission T. These sensors supply respective signals to anelectronic controller of the transmission (not shown).

The operation of the aforementioned individual components of thetransmission will now be described in detail.

The clutch Ci, which enables the internal combustion engine of thetractor to be coupled to the transmission T, is hydraulically operatedby pressurized oil and is therefore disengaged when the engine isstopped. The clutch Ci assists the starting of the internal combustionengine, since it disengages it from the downstream components and theirinertia. When the tractor is stationary and the power take-off isoperating, the clutch Ci disengages the internal combustion engine fromthe transmission T, thus drastically reducing the power dissipated. Theclutch Ci can also uncouple the internal combustion engine from thetransmission, and therefore from the wheels, in an emergency.

The two gearings G1-G2 and G3-G4 drive the hydrostatic unit HU and thetorque splitter unit TSU respectively. In particular, the gearing G1-G2transmits the motion to the variable displacement pump P of thehydrostatic unit HU at a rotational speed which is determined, with aconstant transmission ratio τ₁₂, by the rotational speed of thetractor's internal combustion engine. The gearing G3-G4 transmits themotion to the sun gear B of the torque splitter unit TSU with arotational speed rpmB which is determined, with a constant transmissionratio τ₃₄, by the rotational speed of the internal combustion engine.

As regards the hydrostatic unit HU, assuming that the clutch Ci isengaged and the tractor's internal combustion engine is running at aconstant rotational speed rpmE (corresponding, for example, to themaximum torque speed), the pump 1 is driven by the internal combustionengine through the gearing G1-G2 at a constant rotational speed rpmPequal to rpmE/τ₁₂, and in turn drives the motor M while continuouslyvarying its own displacement. This variation of displacement is obtainedby varying the inclination of a swash plate of per-se-known type whichcarries the pistons of the pump P and which is operated, for example, bytwo proportional solenoid valves which receive from the electroniccontroller appropriate control signals, according to whether the tractordriver wishes to reduce or increase the speed of the vehicle. Therotational speed rpmA of the motor M and of the sun gear A connected toit therefore varies continuously between a value rpmA=rpmP and a valuerpmA=−rpmP as the inclination of the plate varies between a maximumpositive value and a maximum negative value.

Preferably, as shown in the embodiment of FIG. 1, the transmission ratioτ₁₂ of the gearing G1-G2 is equal to the transmission ratio τ₃₄ of thegearing G3-G4 (τ₁₂=τ₃₄=τ), assuming that, at least initially, thevolumetric efficiency of the pump P and motor M unit is equal to 1, sothat the pump P and the sun gear B rotate at the same rotational speedrpmP=rpmB=rpmE/τ. The rotational speed rpmA of the motor M and of thesun gear A connected to it therefore varies continuously between rpmBand −rpmB.

As regards the torque splitter unit TSU, this is driven both by thehydraulic motor M of the hydrostatic unit HU, through the sun gear Awhich rotates at a speed rpmA which is variable (between −rpmB and+rpmB), and by the tractor's internal combustion engine, through the sungear B which rotates at a constant speed rpmB.

The rotational speeds of the sun gears A, B and D and of the planetcarrier C of the unit TSU are correlated with each other as shown by theRavigneaux graph of FIG. 2, in which za, zb and zd indicate,respectively, the numbers of teeth of the planet gears sa, sb and sd,whereas zA, zB and zD indicate, respectively, the numbers of teeth ofthe sun gears A, B and D. Retaining the previous assumption that the sungear B rotates at a constant speed rpmB, while the sun gear A rotates ata speed rpmA varying between −rpmB and +rpmB, the speed characteristicof the sun gear B is represented by the point VB, that of the sun gear Ais represented by the segment lying between the points VA1 (whererpmA=−rpmB) and VA2 (where rpmA=+rpmB), that of the planet carrier C isrepresented by the segment lying between the points VC1 (where rpmC=0)and VC2 (where rpmC=+rpmB) and that of the sun gear D is represented bythe segment lying between the points VD1 (where rpmD=+rpmB) and VD2(where rpmD=k×rpmB, k being a constant depending on the number of teethof the gear wheels of the epicyclic torque splitter unit TSU).

The aforementioned first “work” operating range is obtained by engagingthe clutch Cw of the clutch unit CU and leaving both the other clutch Ctof the unit CU and the braking device BD of the reversing unit RUdisengaged, in such a way that the output shaft OS of the transmission Tis coupled to the second output shaft S6 of the torque splitter unit TSUwhich is drivingly connected to the planet carrier C. In this way, asthe rotational speed of the hydraulic motor M of the hydrostatic unit HUvaries between −rpmB and +rpmB, the rotational speed of the planetcarrier C varies between 0 and +rpmB (as shown in the Ravigneaux graphdescribed above), and therefore the speed of the tractor varies between0 and a value v_work, equal to 20 km/h for example.

At the speed v_work, the plates and counterplates of the clutches Cw andCt rotate at the same speed rpmB, as can be seen from the Ravigneauxgraph described above. In fact, the plate of the clutch Cw, which isdrivingly connected to the output shaft S6 and to the planet carrier Cof the torque splitter unit TSU, rotates at the speed rpmB together withthe associated counterplate to which it is coupled for rotation. Asregards the clutch Ct, the counterplate is drivingly connected to thecounterplate of the clutch Cw and therefore rotates at rpmB, as does theplate which is carried by the output shaft of the unit TSU drivinglyconnected to the sun gear D. It is thus possible to disengage the clutchCw and engage the clutch Ct, thereby obtaining the “transport” operatingrange, in which the output shaft OS of the transmission T is coupled,through the sun gear Br and the shaft S8 of the reversing unit RU, tothe first output shaft S5 of the torque splitter unit TSU which isdrivingly connected to the sun gear D. At this point, as the rotationalspeed of the hydraulic motor M of the hydrostatic unit HU varies between+rpmB and −rpmB, the rotational speed of the sun gear D varies between+rpmB and k×rpmB (as results from the Ravigneaux graph described above),and therefore the speed of the tractor varies between v_work and amaximum value v_max, equal to approximately 45 km/h for example.

A third “reverse” operating range, for moving the tractor in reverse (upto a maximum speed v_rev, of approximately 25 km/h for example), isobtained by operating the braking device BD of the reversing unit RU insuch a way as to lock the planet carrier Cr.

Assuming that the planet carrier is locked, the rotational speeds of thesun gears Ar and Br and of the planet carrier Cr of the reversing unitRU are correlated with each other as shown in the Ravigneaux graph ofFIG. 5, in which zar and zbr indicate, respectively, the numbers ofteeth of the planet gears sar and sbr, while zAr and zBr indicate,respectively, the numbers of teeth of the sun gears Ar and Br. The inputsun gear Ar is drivingly connected to the planet carrier C of the torquesplitter unit TSU and therefore its rotational speed rprAr variesbetween 0 and +rpmB as rpmA varies between −rpmB and +rpmB. The outputsun gear Br, which is drivingly connected to the output shaft OS of thetransmission T, rotates in the opposite direction to the input sun gearAr, thus reversing the output motion, with a rotational speed rpmBrranging from 0 (when rpmAr=rpmC=0) to k′×rpmB (when rpmAr=rpmC=rpmB),where k′ is a constant which depends on the number of teeth of the gearsof the epicyclic unit RU.

The three operating ranges, “work”, “transport” and “reverse”, arerepresented in the graph of FIG. 6, which shows the variation of therotational speeds rpmA (input sun gear A), rpmB (input sun gear B), rpmC(planet carrier C) and rpmD (output sun gear D) as a function of thespeed of the tractor.

The operation of the transmission T is controlled by the aforementionedelectronic controller, which continuously regulates the transmissionratio, and therefore the speed of the tractor, by varying the rotationalspeed of the output shaft of the hydrostatic unit and controlling theengagement and/or disengagement of the clutches, for example accordingto the commands given by the driver by means of per-se-knownelectro-hydraulic devices and according to the information on force andtorque at the tyres and at the internal combustion engine. The signalscorresponding to the rotational speed of the output shaft of thehydrostatic unit, of the input shaft of the transmission and of theoutput shaft of the transmission, read by the sensors ss1, ss2 and ss3respectively, in addition to that of the internal combustion engine(detected by a specific sensor, not shown in the figures), are used asfeedback signals for the closed-loop control of the transmission.

According to another advantageous characteristic of the invention, thehydrostatic unit HU of the transmission T is designed in such a way thatthe maximum speed of the motor M is reached with an inclination of theplate of the pump P which is less (by approximately 10% for example)than the maximum attainable inclination. There is consequently a reserveof angle of inclination of the plate which enables the rotational speedof the hydraulic motor M to be brought to values of less than −rpmB, asindicated by the segment VA1-VA3 shown in a broken line in theRavigneaux graph of FIG. 2.

In this way, with the transmission operating in the “work” range (thatis, with the clutch Cw engaged, the clutch Ct disengaged and the brakingdevice BD of the reversing unit RU disabled), reversing can be carriedout at low speed, up to approximately −2 km/h for example, as shown bythe segment VC1-VCR of the Ravigneaux graph, without the need to use thereversing unit. This operating condition is particularly useful when theposition of the tractor has to be corrected continually, for exampleduring the attachment of transported or towed equipment.

Similarly, with the transmission operating in the “transport” range(that is, with the clutch Ct engaged, the clutch Cw disengaged and thebraking device BD of the reversing unit RU disabled), it is possible toreach a maximum speed of the tractor above the speed v_max (byapproximately 2.5 km/hr for example), by making the transmission operatein the portion of the characteristic defined by the segment VD2-VD3 ofthe Ravigneaux graph, or to reduce the rotational speed of the internalcombustion engine while maintaining the same maximum speed of thetractor so as to limit the fuel consumption and noise level.

As will be clear from the preceding description, the transmissionaccording to the invention offers the advantage of an extremely compactstructure, by virtue of the hydrostatic unit, the torque splitter unit,the clutch unit and the reversing unit being positioned in line along alongitudinal axis spaced downwards from the axis of the shaft of thepower take-off of the tractor. This characteristic of compactness isfurther ensured by the particular construction, without a ring gear, ofthe epicyclic gear train forming the torque splitter unit.

A second embodiment of a hydromechanical transmission according to theinvention will now be described briefly with reference to FIGS. 3 and 4,in which parts and elements identical or corresponding to those of FIGS.1 and 2 previously described have been given the same reference symbols.

The structure of this second embodiment of the transmission T issubstantially identical to that of the first embodiment, except for thetorque splitter unit TSU consisting of an epicyclic gear train with aring gear. In this case, in fact, the torque splitter unit TSU includes:

-   -   a first input sun gear A drivingly connected to the output shaft        S3 of the hydrostatic unit HU;    -   an input planet carrier C drivingly connected to the input shaft        S4 of the unit TSU made in the form of a hollow shaft in which        the shaft S3 is inserted;    -   an output sun gear D drivingly connected to the first output        shaft S5 of the unit TSU;    -   an output ring gear B drivingly connected to the second output        shaft S6 of the unit TSU; wherein the planet carrier C carries:    -   three sets of planet gears sb and sd, spaced apart angularly at        120° (only one of which is shown in FIG. 3), the gears being        drivingly connected for rotation to each other; and    -   three planet gears sa spaced apart angularly at 120° (only one        of which is shown in FIG. 3), each of the planet gears sb        meshing indirectly with the sun gear A via a corresponding        planet gear sa and each of the planet gears sd being interposed        between the sun gear D and the ring gear B.

The speeds of the sun gears and the planet carrier are correlated asshown by the Ravigneaux graph in FIG. 4. By contrast with the firstembodiment, in this case the rotational speed rpmc of the planet carrieris constant, since the planet carrier is driven directly by the internalcombustion engine through the gearing G3-G4, while the rotational speedrpmA of the input sun gear A varies between −rpmC and +rpmc as afunction of the variation of the displacement of the pump P of thehydrostatic unit HU. Consequently, as the rotational speed rpmA of thesun gear A varies between −rpmC and +rpmc, the rotational speed rpmB ofthe output ring gear B varies between 0 and +rpmc (the segment VB1-VB2),thus providing the “work” operating range, in which the speed of thetractor varies between 0 and, for example, 20 km/hr. If rpmA is thenvaried between +rpmC and −rpmC, the rotational speed rpmD of the outputsun gear D varies between +rpmc and k×rpmC (the segment VD1-VD2), wherek is a parameter which depends on the design of the epicyclic unit TSU,thus providing the “transport” operating range, in which the speed ofthe tractor varies, for example, between 20 km/hr and 60 km/hr.

With respect to the solution using a torque splitter without a ringgear, this second embodiment has a smaller overall axial dimension,since it uses a double planet gear sb-sd in place of the triple planetgear sb-sa-sd, but it has a greater overall radial dimension owing tothe presence of the ring gear B. This second solution also providesvalues of k greater than those obtainable in the case of the splitterwithout the ring gear (for example, values equal to or greater than 3,instead of values of approximately 2-2.5), and therefore maximum speedsgreater than those of the solution without ring gear, or alternatively,for the same maximum speed, the possibility of shifting from the “work”range to the “transport” range at a lower speed (for example atapproximately 15 km/hr instead of 20 km/hr). Like the version withoutring gear, this second solution also enables the maximum angular travelof the pump plate to be used to carry out the reverse during operationin the “work” range, without the need to disengage the clutch Cw andoperate the braking device BD.

Clearly, the principle of the invention remaining unchanged, theembodiments and details of construction can vary widely from thosedescribed and illustrated, purely by way of non-limitative example.

1. Continuously variable hydromechanical transmission for agriculturaltractors, comprising: an input shaft (1S) arranged to be coupled to aprime mover of the tractor and extending along a first direction (x1)substantially aligned with the shaft of the prime mover and with a shaft(S1) of the power take-off of the tractor; an output shaft (OS); ahydrostatic unit (HU) including a variable displacement pump (P)arranged to be driven by the prime mover shaft of the tractor and amotor (M) driven by the pump (P), wherein the pump and motor arepositioned substantially in line along a second direction (x2) spacedtransversely downwards from the said first direction (x1); a torquesplitter unit (TSU) of the epicyclic type, positioned substantially inline with the hydrostatic unit (HU) along the said second direction (x2)and including a first and a second input shaft (S3, S4), coupled to themotor (M) of the hydrostatic unit (HU) and to the prime mover of thetractor respectively, and at least a first and a second output shaft(S5, S6), the rotational speeds of the said first and second outputshafts (S5, S6) varying, respectively, in a first and a second range(VD1-VD2, VC1-VC2; VD1-VD2, VB1-VB2) adjacent to each other, at highspeed and low speed respectively, as the rotational speed of the firstinput shaft (S3) varies between a maximum value (rpmA; rpmC) and aminimum value (−rpmA; −rpmC); a clutch unit (CU), positionedsubstantially in line with the hydrostatic unit (HU) and with the torquesplitter unit (TSU) along the said second direction (x2) and arranged tocouple the output shaft (OS) of the transmission selectively to thefirst or second output shaft (5, S6) of the torque splitter unit (TSU),in such a way as to provide a pair of forward operating ranges(“transport” and “work”), at high and low speed respectively; and areversing unit (RU), positioned substantially in line with thehydrostatic unit (HU), the torque splitter unit (TSU) and the clutchunit (CU) along the said second direction (x2), and interposed betweenthe clutch unit (CU) and the output shaft (OS) of the transmission, thesaid unit being arranged to provide a reverse operating range(“reverse”).
 2. Hydromechanical transmission according to claim 1,characterized in that it also comprises a first clutch (C1) for couplingthe input shaft (1S) of the transmission to the shaft of the prime moverof the tractor.
 3. Hydromechanical transmission according to claim 1 or2, characterized in that the input shaft (1S) is made in the form of ahollow shaft and houses within it the shaft (S1) of the power take-offof the tractor.
 4. Hydromechanical transmission according to any one ofthe preceding claims, characterized in that the input shaft (1S) carriesa first and a second driving gear wheel (G1, G3) meshing, respectively,with a third driven gear wheel (G2), coupled to a driving shaft (S2) ofthe pump (P) of the hydrostatic unit (HU), and with a fourth driven gearwheel (G4), coupled to the second input shaft (S4) of the torquesplitter unit (TSU), in such a way that the pump (P) and the shaft (S4)are driven by the input shaft (1S), in other words by the prime mover ofthe tractor, with corresponding constant predetermined transmissionratios (τ₁₂, τ₃₄) through the gearings (G1-G2) and (G3-G4),respectively.
 5. Hydromechanical transmission according to any one ofthe preceding claims, characterized in that the torque splitter unit(TSU) includes: a first input sun gear (A) drivingly connected to thefirst input shaft (S3); a second input sun gear (B) drivingly connectedto the second input shaft (S4); an output sun gear (D) drivinglyconnected to the first output shaft (S5); and a double planet carrier(C) which carries, on an inner circumference, sets of three planet gears(sa, sb, sd) drivingly connected for rotation to each other, namely afirst planet gear (sa), a second planet gear (sb) and a third planetgear (sd), and, on an outer circumference, fourth planet gears (se);each first planet gear (sa) meshing with the first input sun gear (A)via a corresponding fourth planet gear (se), each second planet gear(sb) meshing with the second input sun gear (B) and each third planetgear (sd) meshing with the output sun gear (D); the planet carrier (C)also being drivingly connected to the second output shaft (S6). 6.Hydromechanical transmission according to any one of claims 1 to 4,characterized in that the torque splitter unit (TSU) includes: a firstinput sun gear (A) drivingly connected to the first input shaft (S3); aninput planet carrier (C) drivingly connected to the second input shaft(S4); an output sun gear (D) drivingly connected to the first outputshaft (S5); an output ring gear (B) drivingly connected to the secondoutput shaft (S6), wherein the planet carrier (C) carries: sets of twoplanet gears (sb, sd), drivingly connected for rotation to each other,namely a first planet gear (sb) and a second planet gear (sd), and thirdplanet gears (sa), each first planet gear (sb) meshing with the inputsun gear (A) via a corresponding third planet gear (sa) and each secondplanet gear (sd) being interposed between the output sun gear (D) andthe output ring gear (B).
 7. Hydromechanical transmission according toany one of the preceding claims, characterized in that the reversingunit (RU) includes: a first and a second input shaft (S7, S8); a firstsun gear (Ar) drivingly connected to the said first input shaft (S7); asecond sun gear (Br) drivingly connected both to the said second inputshaft (S8) and to the output shaft (OS) of the transmission; and adouble planet carrier (Cr), which carries on an inner circumference setsof two planet gears (sar, sbr), drivingly connected for rotation to eachother, namely a first planet gear (sar) and a second planet gear (sbr),and, on an outer circumference, third planet gears (sdr), each firstplanet gear (sar) meshing with the first sun gear (Ar), and each secondplanet gear (sbr) meshing with the second sun gear (Br) via acorresponding third planet gear (sdr), the gear carrier (Cr) beingmounted rotatably with respect to the said input and output shafts (S6,S7, OS) and being lockable via a braking device (BD).
 8. Hydromechanicaltransmission according to any one of the preceding claims, characterizedin that the clutch unit (CU) includes: a first clutch (Cw) for couplingthe first and second input shaft (S7, S8) of the reversing unit (RU) toprovide the said low-speed forward operating range (“work”) of thetransmission; and a second clutch (Ct) for coupling the first outputshaft (S5) of the torque splitter unit (TSU) to the second input shaft(S8) of the reversing unit (RU) to provide the said high-speed forwardoperating range (“transport”) of the transmission.
 9. Hydromechanicaltransmission according to any one of the preceding claims, characterizedin that it also comprises an electronic controller for setting therotational speed of the pump (P) of the hydrostatic unit (HU), forengaging or disengaging the clutches (C1, Ct, Cw) and for activating ordisabling the braking device (BD) of the reversing unit (RU) accordingto predetermined operating modes, to provide the aforesaid forwardoperating ranges (“work” and “transport”) and the reverse operatingrange (“reverse”).
 10. Hydromechanical transmission according to claim9, characterized in that it also comprises a first speed sensor (ss1)for detecting the rotational speed of the output shaft (S3) of thehydrostatic unit (HU), in other words that of the first input shaft ofthe torque splitter unit (TSU), a second speed sensor (ss2) fordetecting the rotational speed of the input shaft (1S) of thetransmission and a third speed sensor (ss3) for detecting the rotationalspeed of the output shaft (OS) of the transmission, each of the saidsensors supplying a corresponding signal to the electronic controller ofthe transmission.
 11. Hydromechanical transmission according to any oneof the preceding claims, characterized in that the hydrostatic unit (HU)can supply a rotational speed of the first input shaft (S3) of thetorque splitter unit (TSU), the modulus of which is greater than themodulus of the minimum value (−rpmA; −rpmC), in such a way that when thetransmission is operating in the low-speed operating range (“work”) itis possible to reverse at low speed without the need to operate thereversing unit (RU) and the clutch unit (CU).